Synovitis causes hypoxia and acidity in synovial fluid and subchondral bone.

The purpose of the present study was to evaluate the environmental changes in synovial fluid and subchondral bone during synovitis in rabbits in which the knee joint on one side was subjected to a procedure causing instability; a traumatic synovitis rapidly developed. Three weeks following the procedure, partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2) and hydrogen ion concentration (pH) were measured in vivo in the juxta-articular bone and in the synovial fluid of both the normal and the affected sides. Mass spectrometry was used for simultaneous registration of PO2 and PCO2, while a monocrystalline antimony pH electrode was used for simultaneous measurement of pH in vivo. Hypoxia, hypercapnia and relative acidity were found in synovial fluid and subchondral bone of knees with synovitis. The metabolic environment of synovial fluid and subchondral bone was considerably changed at the 3-week stage of experimental osteoarthritis and this was probably secondary to regional venous congestion.